In a circle, the curvature (or bend) is the reciprocal of the radius,

. (When measured from the inside, the curvature is negative.) Four mutually tangent circles have the property

. This is the Descartes circle theorem. With the centers in the complex plane (

), the following formula holds:

.

Using both formulas, a vast Apollonian packing can be found. This Demonstration focuses on initial circle sets with integer curvature. Hover over a smaller gray circle to see its curvature. Various wonderful results can occur:

1. If the initial curvatures are integers, so are all the curvatures.

2. If the first circle is a line, the configuration is the set of Ford circles.

3. With a large interior circle, the configuration is a Pappus chain.

Frederick Soddy, who won a Nobel prize for his study of isotopes, is better known for his research on circles and a poem he published in

*Nature* in 1936.

The Kiss Precise

by Frederick Soddy

For pairs of lips to kiss maybe

Involves no trigonometry.

'Tis not so when four circles kiss

Each one the other three.

To bring this off the four must be

As three in one or one in three.

If one in three, beyond a doubt

Each gets three kisses from without.

If three in one, then is that one

Thrice kissed internally.